1. Field of the Invention
This invention relates to a novel composition and method for providing insulation for solid propellant rocket motors and, more particularly, to EPDM compositions having fibrous components such as carbon fibers or powder fillers such as silica, or also containing KEVLAR® reinforcing fibers and suitable for internal and external insulation applications on such rocket motors.
2. State of the Art
It is generally accepted current industry practice to prepare insulations for solid propellant rocket motors from a polymeric base importantly composed of an EPDM (ethylene-propylene-diene monomer) terpolymer blend and containing 1,4-hexadiene (HD) as one of the diene monomer components of the EPDM blend.
This EPDM terpolymer, which is commonly designated as the primary EPDM terpolymer since it is present in a higher concentration than the secondary EPDM terpolymer, has been established as a standard for solid propellant rocket motor insulations due to its superior ablation characteristics, excellent physical properties and processability.
For instance, an exemplary carbon fiber-filled rocket motor insulation composed of NORDEL® 1040 as the primary terpolymer is commonly known in the industry as the STW4-2868 thermal insulation and has the following composition as shown in Table 1:
TABLE 1STW4-2868 THERMAL INSULATION FORMULATION(carbon fiber; parts by weight)Parts byIngredientFunctionWeightNORDEL ® 1040Primary EPDM terpolymer base80NEOPRENE ® FBSecondary polymer base20Zinc oxideActivator5SulfurCurative1HAF carbon blackPigment1MBTAccelerator1AGERITE ® Resin DAntioxidant2AGERITE ® HPSAntioxidant1TelluracAccelerator0.50SULFADS ®Accelerator0.75VCM carbon fibersFiller41Total Parts by Weight153.25
Alternatively, solid rocket motor insulations are also composed of compositions employing finely divided powder silica as a filler, with or without the added presence of a fibrous reinforcing agent.
Exemplary silica-filled rocket motor insulations have also included NORDEL® 1040 and NORDEL® 2522 as the primary terpolymer in their formulations and the resulting compositions are respectively commonly known in the industry as the 053A and DL1375 thermal insulations. They have the following compositions shown in Table 2:
TABLE 2THERMAL INSULATION FORMULATION(silica filled; parts by weight)DL1375053A(parts by(parts byIngredientFunctionweight)weight)NORDEL ® 1040Primary EPDM80terpolymer baseNORDEL ® 2522Primary EPDM80terpolymer baseNEOPRENE ® FBSecondary polymer base2020Zinc oxideActivator55SulfurCurative11AGERITE ® Resin DAntioxidant22AGERITE ® HPSAntioxidant11CaptaxAccelerator11TelluracAccelerator0.50.5SULFADS ®Accelerator0.750.75HISIL ® 233Filler35.535.5Total Parts by Weight146.75146.75
In addition, an EPDM terpolymer comprising the HD monomer is sold under the tradename NORDEL® 2722E. An exemplary silica-filled rocket motor insulation comprising NORDEL® 2722E as the secondary terpolymer is commonly known in the industry as the DL1552A thermal insulation and has the following composition as shown in Table 3:
TABLE 3DL1552A THERMAL INSULATION FORMULATION WITH SILICAParts byIngredientFunctionWeightBUNA ® EP T 3950 (Bayer Corp., Fiber, Additives andPrimary EPDM75Rubber Division of Orange, Texas)terpolymer baseNORDEL ® 2722E (DuPont Dow Elastomers)Secondary EPDM20terpolymer base withhigh ethylene contentWINGTACK ® 95 (hydrocarbon resin) (Goodyear Tire andTackifier7Rubber Co., Chemical Division of Beaumont, Texas)IRGANOX ® 1010 (tetrakis[methylene-3-(3′5′-di-tert-butyl-Antioxidant14′-hydroxyphenyl) propionate]methane) (Ciba SpecialtyChemicals, Additives Division, Tarrytown, N.Y.)TRYCOL ® DA-6 (decyl polyoxyethylene alcohol)Wetting agent0.5(Chemical Associates, Inc. of Copley, Ohio)Stearic acid (including palmitic acid) (Harwick StandardCure activator1Distribution Corp. of Akron, Ohio)HISIL ® 233 (silica hydrate) (PPG Industries, Inc. of LakeReinforcing filler45Charles, Louisiana)Aluminum oxide C (Al2O3) (Degussa Corporation ofReinforcing filler0.3Ridgefield Park, N.J.)N330 carbon black (Columbian Chemicals Co. of Marietta,Pigment and1Ga.)reinforcing fillerKALENE ® 1300 (butyl gum elastomer) (Hardman DivisionCo-vulcanizing20of Harcros Chemicals, Inc. of Belleville N.J.)plasticizerHYPALON ® 20 (chlorosulfonated polyethylene) (DuPontCure activator5Dow Elastomers)AGERITE ® Resin D (polymerized trimethylAntioxidant0.25dihydroquinone) (R.T. Vanderbilt Co., Inc. of Buena Park,Ca.)TZFD-88p (zinc oxide dispersed in an EPDM binder)Cure activator2(Rhein Chemie Corp. of Trenton, N.J.)SP 1056 (bromomethyl alkylated phenolic resin)Curing agent15(Schenectady Int'l, Inc. of Schenectady, N.Y.)Total Parts by Weight193.05
An exemplary aramid fiber-filled rocket motor insulation comprising NORDEL® 1040 is commonly known in the industry as R196 thermal insulation and has the following composition as shown in Table 4:
TABLE 4R196 THERMAL INSULATION FORMULATION WITH KEVLAR ®Parts byIngredientFunctionWeightNORDEL ® 1040 (EPDM terpolymer)Polymer base80NATSYN ® 2200 (polyisoprene) (Goodyear Tire andPolymer base20Rubber Co., Chemical Division of Akron, Ohio)WINSTAY ® S (syrenated phenols) (Goodyear Tire andAntioxidant1.0Rubber Co., Chemical Division of Akron, Ohio)Dechlorane Plus 515 (1,2,3,4,7,8,9,10,13,13,14,14-Flame retardant40dodecachloro-1,4,4a,5,6,6q,7,10,10a,11,12,12a-dodecahydro-1,4,7,10-dimethanodibenzo (a,e)cyclooctene) (Occidental Chemical Corporation ofDallas, Texas)Antimony oxide (Sb2O3) (Harcros Chemicals, Inc. ofFlame retardant/filler20Kansas City, Kansas)¼″ KEVLAR ® fiber (aramid staple fiber) (E.I. duPontFiber20de Nemours and Co., of Wilmington, Delaware)VAROX ® DBPH-50 (2,5-dimethyl-2,5-di(t-butylperoxy)Curing agent2.5hexane on a carrier) (R.T. Vanderbilt Co., Inc. of BuenaPark, Ca.)Total Parts by Weight183.5
Numerous past efforts to develop effective replacements for these standard solid rocket motor insulation formulations have not been successful.
The only manufacturer currently producing the foregoing primary EPDM terpolymer in adequate quantities to meet the demands of the rocket motor insulation industry is DuPont Dow Elastomers of Beaumont, Tex., which markets and sells an EPDM terpolymer comprising the HD monomer under the tradename NORDEL® 1040 and NORDEL® 2522.
However, the ability of the industry to produce STW4-2868, DL1375, 053A, DL1552A, R196 and other thermal insulations containing NORDEL® 1040 and NORDEL® 2522, and NORDEL® 2722E as a primary or secondary EPDM terpolymer has recently been placed in jeopardy due to the announcement by DuPont of its intention to cease production of NORDEL® 1040, 2522, 2722E and, generally, other EPDM polymers formed from 1,4-hexadiene. There is, therefore, a need in this industry, previously not satisfied, to find an effective alternate or a replacement for the above-described standard STW4-2868, DL1375, 053A, DL1552A and R196 thermal insulations. Development and formulation of a suitable primary EPDM terpolymer replacement is especially critical for these discontinued NORDEL® insulation formulations.
The requirements for an acceptable, functionally effective, insulation for solid propellant rocket motors are well known to be quite severe due to the extreme conditions to which the insulation is exposed. These conditions not only include exceedingly high temperatures but also severe ablative effects from the hot particles (as well as gases) that traverse and exit the rocket motor interior. Unless the insulation will withstand such conditions, catastrophic failure has and may occur.
U.S. Pat. No. 3,347,047, an early patent describing asbestos fiber-filled insulations, states that flame temperatures encountered in the combustion of propellants, particularly when used as the source of propulsion, necessitating the confinement of the gases of combustion and ultimate release thereof through orifices, are usually accompanied by extremely turbulent flow conditions. All of these features place considerable stress and strain upon the member defining the escape passageway. While the combustion of the propellant in the case of rockets and the like will usually be of short duration, the temperatures and turbulence encountered have been found to very easily destroy even the strongest and most exotic alloys formed of iron, steel, titanium, magnesium, silicon, chromium, beryllium and the like. As a consequence, the projectile structure fails, leading to total destruction thereof through explosion, or in the event that only the exit passageway is destroyed, the projectile proceeds in an erratic, uncontrollable path since its trajectory or path is, at least in part, dependent upon the contour of the passageway through which the gaseous products of combustion pass. That statement still remains fully applicable today.
Therefore, any replacement insulation should exhibit at least comparable temperature-resistant and ablation characteristics and theological and physical properties (e.g., Mooney viscosity) at least equivalent to that of STW4-2868, DL1375, 053A, DL1552A and R196, yet should not otherwise significantly alter the formulation techniques employed for the production of such rocket motor thermal insulation. Additionally, due to the large and growing quantities of solid propellant rocket motor insulation required by the industry, any such replacement EPDM terpolymer candidate should be abundantly available now and into the foreseeable future.
In addition, any replacement EPDM or like terpolymer should satisfy a number of other requirements including wettability of and bond strength with such diverse filler additives as a carbon fiber, aramid fiber, and a silica powder. It is also necessary that such additives be substantially homogeneously dispersed throughout the insulation composition as it is being produced. While standard mixing devices can be employed in the practice of this invention, such as a Banbury mixer, it is a common experience that substantially homogeneous distribution of fibrous additives is not achieved, or achieved only with difficulty, with many elastomeric compositions. Difficulties have been described as in, for instance, during mixing of the components; it can be observed that premature vulcanization may occur as well as other problems that may impede or entirely frustrate effective distribution of the various additives which are essential to the ultimate production of the insulation.
Further, once formulated, the elastomeric composition must also possess acceptable shelf life characteristics such that it remains sufficiently pliable, without becoming fully cured, until used in application to the rocket motor casing. This requirement is essential because the production of a given lot of insulation may have to wait in storage for a number of months prior to use. Typically, the insulation may be stored in large rolls in an uncured or, at most, a partially cured state until ready for use. A number of curing agents are well known and are conventionally employed but still must be compatible with the overall EPDM formulation to permit satisfactory shelf life. This in turn requires a balancing of curing agent activity.
In addition, the formulated insulation should be substantially odorless for obvious reasons and this can require special adjustment of the curing agent components.
After application to the interior (or, if desired, the exterior) of the rocket motor casing and subsequent curing thereof, an acceptable insulation must also exhibit satisfactory bonding characteristics to a variety of adjacent surfaces. Such surfaces include the internal surface of the rocket motor casing itself and the insulation must also exhibit adequate bonding characteristics between itself and the propellant grain, typically with an intermediate liner surface. In turn, the propellant grain in a solid propellant rocket motor is composed of a variety of materials notably including still another elastomer, various combustible materials, and such additional components as aluminum particles.
A functionally acceptable solid propellant rocket motor insulation must meet those requirements and must also survive aging tests. Such rocket motors may be fully fabricated even many months before actual firing and, for tactical weapons especially, sometimes even more than a year or even a plurality of years. For instance, strategic missiles may be stored in silos or submarine launch tubes for decades. Over that period of time, the insulation must continue to remain fully functional without unacceptable migration of its components to or from adjacent interfacial surfaces and adequately retain its elastomeric characteristics to prevent brittleness. This requirement also needs to be satisfied under wide temperature variations. The vibration and physical stress placed on a rocket motor at the time of launch, whether a ground launch or an air firing, is exceedingly high, and brittleness and cracking in the insulation is effectively intolerable, whether from premature or gradual overcure or whatever cause. Even at the end of the burn of the propellant grain within the rocket motor casing, the insulation must remain substantially and functionally intact to avoid potentially catastrophic failures of the entire launch vehicle.
In turn, this means that the insulation composition must meet the ablation limits for protection of the casing throughout the propellant burn without adding undue weight to the motor.
A number of past patents have been granted proposing various solutions to the insulation formulation problem. These include U.S. Pat. No. 3,421,970 (generically describing elastomeric formulations with asbestos); U.S. Pat. No. 3,562,304 (generically describing an elastomeric formulation with asbestos fibers); U.S. Pat. No. 3,637,576 (describing an EPDM formulation with a norbornene component with asbestos fibers); U.S. Pat. No. 4,492,779 (generically describing elastomeric formulations with KEVLAR® fibers); U.S. Pat. No. 4,514,541 (generically a du Pont “master batch” formulation with KEVLAR® fibers, but not an insulation); U.S. Pat. No. 4,550,130 (generically describing a moldable carboxylic acid modified EPDM to enhance affinity to various fillers); U.S. Pat. No. 4,878,431 (generically describing an elastomeric formulation using the EPDM NORDEL® 1040, with KEVLAR® fibers); U.S. Pat. No. 5,364,905 (describing a technique for the in situ polycondensation formation of aramid fibers, but not referring to rocket motor insulations); U.S. Pat. No. 5,498,649 (describing a polyamide/maleic anhydride modified EPDM with KEVLAR® fibers for a rocket motor insulation); U.S. Pat. No. 5,821,284 (a KEVLAR® fiber-filled insulation containing an EPDM illustrated by NORDEL® 2522 in combination with ammonium salts); and U.S. Pat. No. 5,830,384 (generically referring to EPDM's with a “dry water” silica additive for cooling purposes). None of these patents address nor effectively solve the problem faced by the present invention. In fact, the frequent reference to NORDEL® 1040 or NORDEL® 2522 serves to confirm the observation that these particular elastomers are well-nigh the standard in the rocket motor insulation industry.
Accordingly, the search for a functionally satisfactory elastomeric insulation composition requires discovery and implementation of an extraordinarily complex combination of characteristics. The criticality of the material selection is further demonstrated by the severity and magnitude of the risk of failure. Most insulations are of necessity “man-rated” in the sense that a catastrophic failure can result in the loss of human life—whether the rocket motor is used as a booster for launch of the space shuttle or is carried tactically underneath the wing of an attack aircraft. The monetary cost of failure in satellite launches is well-publicized and can run into the hundreds of millions of dollars.
One well-known potential point of failure is the appearance of voids or cracks in the insulation which could lead to the penetration of the rocket motor casing itself. The resultant dispersion of hot gases may not only lead to destruction of the motor generally but can at least lead to its being thrown off its intended course or trajectory with several unhappy results. In such events, the vehicle itself will either self-destruct or be intentionally destroyed, or the satellite will be launched into a useless orbit.
Therefore, one of the most difficult tasks in the solid propellant rocket motor industry is the development of a suitable, acceptable insulation composition that will meet and pass a large number of test criteria to lead to its acceptability.
Furthermore, any replacement EPDM terpolymers should not be susceptible to obsolescence issues nor discontinuance in future supply thereof.